The present invention relates to a curable epoxy resin composition or, more particularly, to a curable epoxy resin composition suitable for encapsulating electronic devices such as semiconductor-based ones, e.g. transistors, ICs (integrated circuits), LSIs (large-scale integrated circuits) and the like.
Epoxy resin-based molding compositions have found wide applications in a variety of industrial fields and, in particular, they are useful materials in the electronics industry owing to their excellent heat curability, electric properties and adhesive bonding strength in addition to the inexpensiveness in comparison with the other kinds of thermosetting resin-based molding compositions. In particular, epoxy resin-based molding compositions in general exhibit sufficient flow even under a relatively low pressure in the molding so that they are advantageously used in encapsulating various kinds of electronic devices or semiconductor devices such as transistors, ICs, LSIs and the like without injuring the devices by the techniques of low-pressure transfer molding to give high productivity.
Needless to say, these epoxy resin-based molding compositions are formulated with a curing agent and typical curing agents include anhydrides of carboxylic acids, aromatic amine compounds, novolac-type phenolic resins and the like with their respective advantages and disadvantages. In particular, epoxy resin-based molding compositions formulated with a novolac-type phenolic resin are advantageous in respects of excellent moldability and anti-moisture resistance in addition to the outstanding inexpensiveness and absence of toxicity when compared with the compositions formulated with the other kinds of the curing agents so that they are the most widely used resin compositions for encapsulation of semiconductor devices.
One of the disadvantages to the contrary in the epoxy resin-based molding compositions formulated with a novolac-type phenolic resin is the poor electric properties of the cured products thereof at elevated temperatures. For example, a MOS type semiconductor device having a relatively high working temperature of 80.degree. C. or higher sometimes works wrongly exhibiting no normal performance of the semiconductor when encapsulated with the epoxy resin composition of this type due to the increased leak current between the electrodes. Further, ICs encapsulated with the resin composition are defective because breaking of the wiring in the chips takes place at a relatively early stage when they are used at a high temperature and under a high humidity by the corrosion of the aluminum wires used for the wiring.
Therefore, one of the most important problems in the technology of the encapsulation of semiconductor devices by use of a resin composition is the improvement of the high temperature performance of the epoxy resin-based compositions formulated with a novolac-type phenolic resin as the curing agent.